A Queensland principal who was sacked for not telling police about reports of a teacher molesting students says it is hard to come to terms with how he handled the issue.

Teacher Gerard Vincent Byrnes molested 13 female students, all aged between nine and 10, at a Catholic school in Toowoomba in 2007 and 2008.

The Royal Commission into Institutional Responses to Child Sexual Abuse is looking at why Byrnes was allowed to go on offending for more than a year after allegations were first made against him to the school.

Byrnes, the school's child protection contact, pleaded guilty in 2010 to child sex offences, including rape. He carried out all but two of his crimes in the classroom.

Former principal Terence Hayes today accepted that his failure to report the details of the most serious sexual allegations was "grossly incompetent".

He conceded he did not tell the officers about the most serious allegations, including a student's 2007 claim Byrnes put his hand up her skirt.

"It's an oversight that I find very hard to come to terms with. It brings a lot of hurt to me and my family," he said.